Systemic lupus erythematosus (SLE) is a debilitating, heritable autoimmune disease, with a multitude of clinical manifestations including pathogenic autoantibody production and multi-organ damage. SLE disproportionately afflicts women (9-fold higher than men) and ethnic populations. Asians and African-Americans have higher SLE incidence, more severe disease manifestations and greater risk of organ damage (i.e., lupus nephritis) than Caucasians. SLE has a strong genetic component. Our recent study on Asian populations reported several novel and re-confirmed previously known SLE susceptibility loci. These include two guanine nucleotide exchange factors, RASGRP1 (p=4.7x10-10) and RASGRP3 (p=8.7x10-10). RASGRP1-3 play pivotal roles in regulating differentiation, proliferation and apoptosis of lymphocytes (i.e., B-, T-cells), which are highly dysregulated in SLE. In mice, deficiency of these proteins results in lupus-like autoimmune disease (Rasgrp1) and immune dysfunction (Rasgrp3). Despite the strong association of these genes as a whole, specific pathogenic functional variants and their molecular mechanisms are yet to be defined. Bioinformatics data predicted that several associated variants, located in important regulatory regions, are cis-eQTLs, implicating their regulatory roles in expression. We experimentally validated the allele-specific regulatory effects of some of these predicted functional variants. Since SLE is 3-5 times more prevalent in individuals of non-European ancestry, we propose a comprehensive, sequence-based trans-ethnic mapping approach followed by experimental validation with functional genetics including CRIPR/Cas9. This approach will be informative in identifying causal variants and their molecular mechanisms, and understanding clinical heterogeneity across patients of multiple ethnicities. Our research team has the expertise, resources and infrastructure necessary to move beyond GWAS and accelerate the discovery and characterization of the functional variants, as we did successfully in ITGAM, NCF2, BLK and IFIH1. We propose two Aims. In Aim 1, we will localize additional SLE-predisposing variants from RASGRP1-3 by performing comprehensive imputation-based trans-ethnic mapping across four ethnically diverse populations (N>20,000 from Asian, African-American, European-American, and Hispanic descent). Promising variants, especially imputed and low-frequency variants, will be validated through additional confirmatory genotyping. We will also elucidate genetic and clinical heterogeneity by assessing the association of variants with clinical sub- phenotypes and autoantibodies for SLE. In Aim 2, we will predict and experimentally validate the mechanistic effects of the putative variants in both cultured cells and primary immune cells (B- and T-cells) from patients and controls. The knowledge gained from this project will provide mechanistic insights for defining how RASGRP1-3 risk alleles are involved in SLE pathogenesis.